Characterization of bismuth–cerium-molybdate selective propylene ammoxidation catalysts

• Structural characterization of a bismuth–cerium molybdate acrylonitrile catalyst.
• Single phase bismuth–cerium molybdate is a defect scheelite-derived structure.
• Bismuth occupies a selective crystallographic site in the cerium molybdate structure.
• The catalytically active site structure for propylene ammoxidation is given.

Acrylonitrile is a major chemical intermediate used in the production of a wide range of chemical and polymer products. Central to the commercial process is a proprietary catalyst consisting of a complex mixture of metal oxides containing a bismuth-containing molybdate phase that is active and selective for propylene ammoxidation to acrylonitrile. Among the most active and selective is a solid solution of bismuth and cerium molybdate. Solid state structural studies were undertaken to characterize this active phase. The results show that the mixed bismuth–cerium molybdate consists of a solid solution phase having the scheelite-related structure of cerium molybdate with a monoclinic unit cell. Analysis of the cation site occupancy using synchrotron X-ray diffraction indicates that bismuth preferentially occupies the Ce(3) site of the monoclinic cerium molybdate structure. It is therefore possible to singularly identify the structure of the active site for propylene ammoxidation given that bismuth is a necessary constituent of a site for selective propylene (amm)oxidation. The proposed active site consists of bismuth located next to a cation vacancy in the structure, presumably in order to accommodate its lone pair of electrons. Bismuth serves as the site for the rate determining α-hydrogen abstraction from propylene to form an allyl intermediate and subsequent nitrogen insertion and loss of lattice oxygen. The bismuth site is surrounded by two cerium cations in this active site configuration. Thus the model that emerges from this study is Bi3+ and Ce3+ in a molybdate structural framework with cerium readily able to undergo Ce3+ ↔ Ce4+ redox that facilitates lattice oxygen transfer to the active site as required by the operative Mars–van Krevelen mechanism for selective propylene ammoxidation. The presence of two cerium cations adjacent to bismuth as a key component of the active site is expected to promote the rapid re-oxidation of the catalytic site effecting enhanced catalytic performance with respect to selective product yields and productivity.

Proposed model of active sites for propylene ammoxidation in bismuth–cerium molybdate catalyst having the cerium molybdate structure.Figure optionsDownload high-quality image (195 K)Download as PowerPoint slide